1. Field of the Invention
The present invention relates to high speed automatic wire bonders and to wire bonding monitoring systems. More specifically, the present invention relates to a high speed detecting circuit for detecting the touch down of a bonding tool onto a bonding target prior to performing a bonding operation.
2. Description of the Prior Art
Automatic wire bonders are designed to make over ten distinct bonds per second on the pads of a semiconductor chip or the fingers of lead frames or on chip carriers. The targets to be bonded vary in height by several one thousandths (Mils) of an inch. The bonding tool at maximum velocity is being driven toward the bonding target at speeds up to 25,000 Mils per second. In order to avoid crashing the bonding tool into a bonding target, it has been heretofore the accepted practice to perform a teach operation in which a touch down Z-axis position is determined for all bonding targets to be bonded. The automatic wire bonder is then employed to calculate a Z-axis position above all of the touch down positions commonly known in this industry as an inflection point. When the bonding tool is driven toward a bonding target, it passes through the predetermined inflection point at which time the velocity of the bonding tool is changed to a lesser and controlled constant velocity which continues until the time of impact and touch down occurs. This impact/touch down velocity is controlled to avoid producing a destructive impact force or a force which would affect the quality of a bond on a semiconductor device.
The bonding force employed to affect a proper first or second bond is actuated after impact force and touch down, however, ultrasonic energy imparted to the bonding tool may be started before or after touch down. Different problems presented by different semiconductor chips and lead frames and carriers require that present day automatic wire bonders be capable of bonding all different types of semiconductor chips and lead frames using bonding parameters determined during setup and/or teach operations.
Most operators of automatic wire bonders attempt to produce the strongest possible wire bonds in a minimum or nominal time for making an interconnection while maintaining maximum continuous operation before assistance of the bonding machine. The result determines the maximum throughput of acceptable products. Our U.S. Pat. Nos. 4,266,710; 4,586,642, 4,808,948 and 5,111,989 show and describe methods and means for controlling such wire bonding operations to effect maximum throughput. These patents are assigned to the same assignee as the present invention and are generally classified in U.S. Class 228, Sub-Classes 4.5, 1.10, 1.79, etc. As will be explained in greater detail hereinafter such prior art wire bonders have a built-in safety factor and reduce the maximum approach velocity of the bonding tool well before impact on the bonding target.
It would be highly desirable to provide a method and means for sensing the touch down point of a bonding tool on a bonding target at speeds that are so high that the detrimental effect of high speed impact forces can be controlled after impact which was not possible heretofore.